Field of the Invention
The invention relates to a method for knock regulation in an internal combustion engine, wherein knock values are formed from signals of at least one knock sensor within a measurement window to be specified for each ignition, the knock values are used to form noise values, the noise values are used to form knock thresholds with which the knock values are compared, and engine knocking is identified if the knock values exceeds the knock threshold.
The starting point for knock regulation of a multicylinder internal combustion engine is a knock threshold KS for each individual cylinder (which is smaller than or equal to a specified maximum knock threshold KS.sub.max applicable to all of the cylinders), which is generally formed by knock sensor signals or by knock values KW determined for that cylinder from those knock signals (which is referred to below as knock values) and with which these knock values are compared. If the knock threshold KS is exceeded by the knock values KW, engine knock is inferred and, for example, the ignition point for this cylinder is retarded. If the knock threshold is not exceeded for a prolonged period, the ignition point is slowly advanced again.
If knock values assume a high value several times in succession, the knock threshold that is calculated from them continues to be increased, thereby requiring ever higher knock values to exceed that threshold. Knock regulation becomes less sensitive (engine knock is not detected) and the risk of engine damage increases.
European Patent Application 0 399 068 A1, corresponding to U.S. Pat. No. 5,040,510, has disclosed a cylinder-selective knock-regulating method in which sensor signals from a knock sensor are sampled at a specified sampling frequency in a particular angular range of the engine crankshaft, within a measurement window that can be specified for each cylinder. In that case, a specified number of the highest sample values from each measurement window is used to determine a knock value KW, by averaging for example, and a specified number of previous knock values KW is used to calculate a variable noise value GW, by sliding average formation, for example. A knock threshold KS is determined from that value, e.g. KS=(GW+S)*F or KS=GW*F+S, wherein S is a knock summand and F is a knock factor. Both variables are either specified values or are stored in tables as a function of at least one operating parameter (engine speed n, load L, etc.), preferably in characteristic maps as a function of engine speed n and load L. Both variables are used to fine-tune the knock threshold to ensure good knock detection, even with difficult boundary conditions, e.g. a poor signal-to-noise ratio at very low engine speeds.
The noise value GW can be determined more simply from the current knock value KW and the previous noise value GW, which implicitly contains the preceding knock values.
In order to prevent knock regulation becoming increasingly insensitive, as described above, provision is made in the case of the subject matter of European Patent Application 0 399 068 A1, corresponding to U.S. Pat. No. 5,040,510, not to use knock values that exceed a specified value for the calculation of the noise value GW.
If those knock values are included in the calculation with the value "0", then, in the case of intensive knocking, the knock threshold may become lower and lower and engine knock may be detected more and more often even though engine knock is not occurring. In that case, the ignition point may be retarded to an ever increasing extent.
However, if, as is more sensible, only knock values which have been assessed as free from knock are used, the knock threshold KS may be increased to an ever greater extent by a number of knock values KW which are just below KS, and knock regulation becomes increasingly insensitive.